1. Field of the Invention
This invention pertains generally to plasma sources that ionize gas and heat plasma using RF antennas, applied magnetic fields, plasmas and coaxial gas confinement tubes. Applications comprise plasma production for doping and testing materials in plasma processing applications, break-down of toxic gas streams, and sterilization of materials by bombardment with plasma. Space applications comprise using these systems for rocket engines to provide thrust by discharging ionized particles in a particular direction. More specifically, the invention relates to improvements to antennas, magnet geometries, and thermal solutions for the steady state production and heating of plasmas.
2. Description of the Related Art
The application that covers the broadest implementation of these arts is the Variable Specific Impulse Rocket (VASIMR). The VASIMR is fundamentally an electromagnetic plasma accelerator that borrows heavily from the physics and technology of magnetic confinement fusion research. In the jargon of the fusion community it is called a “magnetic mirror,” an open magnetic system or “linear machine” because unlike the toroidal Stellerator or Tokamak, the topology of the magnetic field is open. Linear machines were pursued heavily in the 1970s as potential fusion devices, but due to their weak axial plasma confinement, lost favor in the United States over closed topology magnetic systems such as the Tokamak and the Stellerator. The weakness of the mirror machine as a plasma confinement device is the strength of the VASIMR as a plasma thruster. Plasma in these systems is radially confined, but free to flow axially out of the device to provide rocket propulsion. Therefore, the bulk of the prior art on open ended magnetic confinement systems could be relevant to furthering the technology of the VASIMR.
Three linked magnetic stages perform specific interrelated functions in VASIMR. The first stage handles the main injection of propellant gas and its ionization; the second, also called the “RF booster” acts to further energize the plasma; the third stage is a magnetic nozzle, which converts the energy of the fluid into directed flow. VASIMR is a radio frequency (RF) driven device where the ionization of the propellant is done by a helicon-type discharge. The plasma ions are further accelerated in the second stage by ion cyclotron resonance heating (ICRH), a well-known technique, used extensively in magnetic confinement fusion research.
It is known in the art that plasma may be accelerated by a series of antennas to generate thrust in a rocket engine. U.S. Pat. No. 6,334,302 describes a variable specific impulse magnetoplasma rocket (VASIMR) using two antennas to deliver energy to a gas stream. First, a helicon antenna is used as part of a helicon plasma generator to impart radio frequency (RF) power to the gas stream exciting the gas atoms to an ionized state.
Downstream of the helicon antenna, the resulting plasma is subjected to additional RF power imparted by an Ion Cyclotron Radio Heating (ICRH) antenna to excite ion cyclotron resonance on the plasma. The power imparted to the plasma by the antennas is converted to kinetic energy when the ions are thereafter ejected through a magnetic nozzle to provide the desired thrust.
Overall system efficiency, neglecting any ambipolar contribution to thrust, can be expressed as a ratio of exhaust kinetic energy to electrical power input, where part of the electrical power input goes to the helicon plasma generator and part goes to ICRH antenna. Lower mass flow has higher flow velocity, demonstrating variable specific impulse control technique.
In a VASIMR rocket, neutral gas is first injected into a tube with RF compatible dielectric properties. As the gas flows downstream, plasma is generated as the gas is ionized by a helicon antenna. At this stage the temperature of the plasma may be about 60,000 Kelvin. As the plasma flows further downstream it is further heated by an ion cyclotron resonance heating (ICRH) antenna where it could reach temperatures in the millions of Kelvin. The engine's surrounding surfaces are protected from direct contact high temperature plasma by a magnetic field acting on the plasma. However, considerable heat is still transferred between the hot plasma and the antennas, primarily 15 through radiation from the plasma.
A heat pipe is a passive device for heat removal. The extremely high temperatures associated with the plasma require that the plasma be contained by a magnetic field. Heat transferred by radiation or other mechanisms from the plasma to the surrounding surfaces must be removed in steady-state operation if the surrounding surfaces are to maintain their structural integrity.
The use of a heat pipe to transfer heat efficiently from a hot location to a cold location is known in the art (See U.S. Pat. No. 2,350,348). Generally a heat pipe consists of a vacuum tight envelope, a wick structure and a working fluid. The heat pipe is evacuated and then back-filled with a small quantity of working fluid, just enough to saturate the wick. The atmosphere inside the heat pipe is set by an equilibrium of liquid and vapor. As heat enters the heat pipe at the hot end, (the evaporator), this equilibrium is upset generating vapor at a slightly higher pressure. This higher pressure vapor travels to the cold end (the condenser) where the slightly lower temperatures cause the vapor to condense giving up its latent heat of vaporization. The condensed fluid is then pumped back to the evaporator by the
capillary forces developed in the wick structure.
The present inventors are aware of no prior art where thermal management of a helicon—especially antennas and ionization chamber tubing—is achieved through innovative use of heat pipes, coolant flow, heat exchangers, and/or thermally conductive materials with low RF dielectric losses such as CVD diamond. The present inventors are aware of no prior art where gas density and flow rate, antenna design, and magnetic field shape, including the choke design, are all optimized to move the hot plasma downstream for steady-state plasma source operation.